Prostaglandin E1 (PGE1), a cyclic derivative of prostanoic acid, has been associated with a variety of effects on tissues including stimulation of Na ion transport in epithelial tissues. Human red cells appear to lack adenyl cyclase which is thought to be involved with Ca 2 ion in ion transport in epithelial tissues. Ca ions are known to stimulate the hydrolysis of ATP in the presence of Na plus K ions plus ouabain in human red cell stroma. In these experiments maximum ATP hydrolysis with stroma occurred at about 0.15 mM external Ca 2 ion and was at a rate several times greater than with Na plus K ions alone. In the presence of PGE1 maximum hydrolysis of ATP occurred in the presence of Na plus K ions plus ouabain when only residual stromal preparations containing ouabain alone, PGE1 stimulated to a lesser extent and was maximal between 0 and 0.1 mM added Ca 2 ions. When reconstituted ghosts were prepared to contain Na 24 and ATP and were examined in presence of PGE1, Na 24 efflux was less in PGE1-treated ghosts than in those without PGE1. This inhibition of Na 24 efflux occurred at PGE1 concentration of 10-8 M. Other experiments indicate that the PGE1 effect occurs only when ATP is present in the ghosts.